This invention relates to light emitting devices formed by using an AlGaInP-based compound semiconductor material for emitting visible light. More particularly, this invention relates to a semiconductor light emitting device having a semiconductor crystal layers improved in film quality to enhance electric characteristics such as light emitting efficiency.
A conventional semiconductor light emitting device for emitting visible light has a light emitting layer forming portion formed by employing an AlGaInP-based compound semiconductor, as shown in FIG. 5. In FIG. 5, an n-type GaAs substrate has thereon a light emitting layer forming portion 29 of a doublehetero junction structure. That is, the light emitting layer forming portion 29 is formed by epitaxially growing, in order, an n-type cladding layer 22 of an n-type AlGaInP-based compound semiconductor material, an active layer 23 of a non-doped AlGaInP-based semiconductor material having a composition lower in bandgap energy than that of the cladding layer, and a p-type cladding layer 24 of a p-type AlGaInP-based semiconductor material. On the surface of the light emitting layer forming portion 29, a p-type GaP window layer (current diffusion layer) 25 is further epitaxially grown. A p-side electrode 27 and an n-side electrode 28 are respectively formed of an Au--Zn--Ni or Au--Ge--Ni alloy at main and back surfaces.
The light emitting device of this structure emits utilizable light through a surface of the semiconductor layers, i.e. the surface having the p-side electrode 27. Consequently, the p-side electrode 27 as a block to light has to be provided at an area as small as possible. On the other hand, since light emission is made by confining carriers within the active layer 23 sandwiched between the cladding layers 22, 24, electric currents have to be uniformly distributed over the entire light emitting layer. For this reason, the window layer 25 is provided for spreading the electric currents throughout an entire plane within the chip.
It is desired that this window layer 25 is formed so as to sufficiently diffuse electric currents and at the same time nonabsorbent of the light emitted from the active layer 23. There are some cases that a material GaP having a large bandgap energy is employed in order to increase spread of electric currents. There are other cases, as in an example of FIG. 6, that the window layer is formed by a composite layer having an AlGaAs-based compound semiconductor sublayer with high carrier concentration and a GaP sublayer with somewhat lower carrier concentration but large in bandgap energy so as to reduce light absorption.
In the conventional semiconductor light emitting devices as shown in FIGS. 5 and 6, lattice matching between the GaAs substrate and the AlGaInP-based compound semiconductor (lattice constant: 5.653 angstroms) is made through controlling a crystal mixture ratio of (AlGa) and In. Further, lattice matching is given also to an AlGaAs-based compound semiconductor. However, lattice matching is not given to between the AlGaInP-based compound semiconductor or AlGaAs-based compound semiconductor and the GaP material (lattice constant: 5.451 angstroms). Accordingly, there is a problem that the GaP window layer suffers deterioration in film quality and has an increased electric resistance to thereby increase operating voltage or weaken current diffusion, resulting in lowering in electric characteristics such as light emitting efficiency.
Meanwhile, in the semiconductor light emitting device structured as shown in FIG. 5, the window layer has to be formed at a high carrier concentration and an increased thickness in order to diffuse electric current. However, there is a limitation in increasing the carrier concentration by impurity doping. It is impossible to increase the carrier concentration to such a level as obtainable in metals. In order to spread the current supplied from the upper small electrode to the entire plane of the device chip before reaching the cladding layer, there is a necessity of increasing the thickness of the window layer to approximately 10-60 .mu.m as in the conventional. Such a thick window layer requires a long time, approximately 12-20 hours, of epitaxial growth. Even if the layer thickness is achieved, it is difficult to diffuse electric current to a sufficient extent. Current distribution is uneven to cause deviated light emission with reduced light emitting efficiency. Further, the GaP window layer is not given lattice matching. Accordingly, if the thickness of the window layer is increased, the film quality thereof is deteriorated to further increase the series resistance, raising a problem of lowering electric characteristics such as increase in operating voltage and lowering in light emitting efficiency due to insufficient current diffusion.
Furthermore, although the GaP layer is large in bandgap energy, it absorbs the light emitted by the light emitting layer to a certain extent, and is not perfectly transparent for visible light.
It is therefore an object of the present invention to provide a semiconductor light emitting device having a light emitting layer formed of an AlGaInP-based compound semiconductor and a window layer formed of a material such as GaP having a large bandgap energy, which is improved in light emitting efficiency and electric characteristics without degrading the film quality of the window layer.
It is another object to provide a semiconductor light emitting device which is suppressed against light absorption by the window layer, enhancing the efficiency of light radiating outward the device.
It is further object of the present invention to provide a semiconductor light emitting device having a light emitting layer forming portion formed of an AlGaInP-based compound semiconductor and a window layer formed of GaP, which is capable of diffusing electric current sufficiently for enhancing light emitting efficiency while formed in a shortened growth time by reducing the thickness of the window layer.